Example Load Calculations Using the Standard Method: Given:
  1. Single family dwelling with a living floor area= 3700 s.f.
  2. No. of small appliance branch circuits=5
  3. No. of laundry branch circuits=2
  4. Dishwasher: 12 AMPS, 120V
  5. Water heater: 5 kw, 240V
  6. Garbage disposal: 1/2 hp, 115V
  7. Trash compactor: 10 AMPS, 120V
  8. Attic fans: 3 @ 5 AMPS each= 15 AMPS, 120V
  9. Clothes dryer: 6 KW, 240V
  10. Counter-mounted cooking unit (cooktop): 7 KW, 240V
  11. Wall-mounted oven: 6 KW, 240V
  12. Air handler (blower motor): 4 AMPS, 115V
  13. Air conditioning compressor: 18 AMPS, 230V
  14. Condenser fan motor: 3 AMPS, 115V
  15. Space Electric heat: 16 KW, 240V (calculated @ 100%) of connected load

Answers;
(inputs and calculated values are in red)

STANDARD METHOD-SINGLE FAMILY DWELLING
ELECTRICAL SIZING CALCULATIONS
1. General Lighting & Receptacle Loads. NEC 220-3(b)
Not including open porches, garages, unused or unfinished spaces not adaptable for future use
3 VA x
 3700
(sq. ft.)
1 11,100
2. Small Appliance Branch Circuits. NEC 220-16(a)
Al least 2 small appliances branch circuits must be included. NEC 210-11(c)1
1500 VA x
 5
(min. 2)
2 7,500
3. Laundry Branch Circuit(s). NEC 220-16(b), 220-4(c)
At least 1 laundry branch circuit must be included.
1500 VA x
2
(min. 1)
3 3,000
4. Add lines 1, 2 and 3 4 21,600

5. 21,600 (line 4)    -  3000  = 18,600  VA
   
5 18,600 

6. 18,600 (line 5)    x   0.35=  6,510 VA
  
6 6,510
7. TOTAL General Lighting and Receptacle Load

3000 + 6,510 (line 6)  = 9,510 VA
                                          
7 9,510
8. Fastened-In-Place  Appliances. NEC 220-17
Use nameplate rating. Do not include electric ranges, clothes dryers, space-heating or A/C equipment
 

water heater: 5,000 (in VA's)  [kw=1000 va]

 

compactor:    10A x 120V= 1,200 

 

dishwasher: 12A x 120V= 1,440

 

garbage disposal: 
1/2hp=9.8 , from NEC Table 430-149
VA= 9.8A x 115V= 1,127

 

attic fans: 15A x 120V= 1,800

If fewer than 4 units, place total volt-amps (add the values) on line 8.
Or, 

If 4 or more than units
, multiply total volt-amps x 75%,

[5,000+1,200+1,440+1,127+1,800] x 0.75 = 7,925 , enter on line 8

8 7,925
9. Clothes Dryers. NEC 220-18
If not present >> skip. Otherwise, use 5000 watts or nameplate rating whichever is larger
9 6,000
10. Ranges, ovens, Cooktops and other cooking appliances over 1750 watts. NEC 220-19. If not present >> skip. Otherwise, use Table 220-19 along with notes
A 7 kw cooktop and 6kw wall oven fall within col. "C" in Table 220-19. Demand factor= 65%. Hence, (7kw+ 6kw) x 0.65= 8.45 kw, or 8450 watts (volts-amps)
10 8,450
11. Heating or Air Conditioning. NEC 220-19.
  1. Compare between heat and cooling- eliminate the smaller.
  2. Include the air handler when using either one. 
  3. For heat pumps, include the compressor and the max. amount of electric heat which can be energized with the compressor running

Heating: 16 kw= 16,000 va + 4A x 115V= 460 va (air handler-blower)= 16,460 

Cooling: 18A x 230V (compressor) + 3A x 115V (condenser fan motor) + 460 (blower motor)= 4140+345+460= 4,945

Heating load governs @ 16,460


11 16,460
12. Largest motor (only one). NEC 220-14, 430-24
Multiply the largest motor volt-amps x 25%

Usually the air-conditioner compressor is the largest motor. But since that appliance load was omitted, see step 11 above, use the next largest motor. In this case, the garbage disposal @ 1127 va. So 1127 x 0.25= 282 
12 282
13. TOTAL Volt-Amp Demand Load: Add lines 7 through 12 13 48,627
14. MIN. Amperes for Service (Total v-a / voltage)=

( Line 13) / 240V (voltage= 240, for 120/240 V service)=

48,627 / 240V=  202 AMPS


14 202

MIN. amps
15. MIN. Size for Service Conductors/Feeders. NEC 240-6(a). Use next STANDARD size fuse or breaker . 15 225 AMPS
16. Size of Service and/or Feeder Conductors
Use NEC 310-15(b) for the service conductors up to 400A.
Rating > 400 amps; Use Table 310-16 . MIN. SIZE of CONDUCTORS=
16 Use 3/0 CU, 

OR,

250 kcmil AL
17. Size of Neutral Conductor. NEC 220-22
  1. NEC 310-15(b) allows neutral to be smaller than the ungrounded (hot) conductors, provided 215-2, 220-22 and 230-42 are met. 
  2. Size the neutral by finding the max. unbalanced load
  3. Neutral cannot be smaller than the required grounding electrode conductor specified in Table 250-94 . MIN. SIZE of NEUTRAL CONDUCTOR=
17 Use No. 2 CU

See
below for neutral sizing calcs)
20. Size of Grounding Electrode Conductor (GEC). NEC, Table 250-94 . MIN. SIZE of GEC:

Based on 225 AMP service with 3/0 service conductors, the size of the GEC is No. 4 per Table 250-94
18 No. 4 CU

Sizing the Neutral Conductor:

Need to calculate all loads from appliances that contribute to the neutral. The neutral is sized by finding the max. unbalanced load.

  1. General lighting & receptacles= 9,510 (from line 7 above)
  2. Fastened-in-place appliances:
    Water heater= 0 (no neutral conductor connected, 240V)
    Dishwasher= 1,440
    Garbage disposal=1,127
    Compactor= 1,200
    Attic fans= 1,800
    TOTAL =5,567
    Factored TOTAL= 5,567 x 0.75= 4,175 (NEC 220-17)
  3. Clothes dryer= (Line 9) x 70%= 6,000 x 0.7=4,200
  4. Cooking appliances= (Line 10) x 70%= 8,450 x 0.7= 5915
  5. Electric heat and air conditioning= 0 (240V)
  6. Air handler= 460
  7. Condensing unit= 345
  8. Largest motor= 282

GRAND TOTAL= 9,510+4,175+4200+5915+460+282= 24,542 VA

AMPERAGE= 24,542 / 240V= 102 AMP

A No, 2 CU (75) is good for 115 AMP per Table 310-16

Hence the minimum size for the neutral shall be a No. 2 CU .

Note! The neutral conductor, No. 2 CU, is smaller than Service-Entrance Conductors, No. 3/0, and larger than the Grounding Electrode Conductor, No. 4 CU. OK.

v-a= volts-amperes=watts
To find amperage of 1-phase appliances when HP and voltage is known use Table 430-148